Unreleased vocals.....but after awhile they just used the acapellas from his recorded songs and put them over new beats from newer producers that 2Pac never actually worked with. Like that album Eminem produced.

And on some of them they started adding guest verses with rappers Pac never worked with.

The earlier posthumous stuff that came out shortly after his death, they mostly didn't mess with and just put it out the way Pac recorded it.

So they would take a verse from hail Mary and put it on a new beat? (Is that what you mean by acapellas from recorded songs?)